Multilayer filter medium for use in filter, and filter

ABSTRACT

A multilayer filter medium, which is used as a constituent member of a filter and has a multilayer structure, includes a wet type nonwoven fabric layer A containing 0.5 to 20% by weight, based on layer weight, of short-cut nanofibers which are composed of a fiber-forming thermoplastic polymer, have a single fiber diameter (D) of 100 to 1,000 nm and are cut so that the ratio (L/D) of the length (L) to the single fiber diameter (D) is within the range of 100 to 2,500 and core-sheath conjugate type binder fibers having a single fiber diameter of 5 μm or more, and a nonwoven fabric layer B having a lower density than that of the wet type nonwoven fabric layer A. A filter uses the multilayer filter medium and has the nonwoven fabric layer B arranged on a fluid inlet side.

TECHNICAL FIELD

The present invention relates to a multilayer filter medium for use in afilter, which makes it possible to obtain a filter having highcollection efficiency, low pressure loss and a long filter lifetime, andto a filter using the multilayer filter medium for use in a filter.

BACKGROUND ART

As filter media for use in filters, various ones have hitherto beenproposed. For example, there have been proposed an airlaid multilayerfilter medium having a fiber fineness gradient (for example, see PatentDocument 1), one in which ultrafine fibers obtained by anelectrospinning process are laminated on a surface layer of ageneral-purpose nonwoven fabric (for example, see Patent Document 2 andPatent Document 3), and the like.

However, the airlaid multilayer filter medium having a fiber finenessgradient has achieved low pressure loss and a high filter lifetime, buthas been insufficient for collecting fine dusts. Further, the filtermedium in which the ultrafine fibers are laminated on the surface layerof the general-purpose nonwoven fabric comes into a state planarlycoated with the ultrafine fibers, so that there has been a problem of aneasy increase in pressure loss, easy omission of the fibers because oftheir insufficient adhesiveness with the nonwoven fabric as a substrate,or the like.

Incidentally, there has also been proposed a nonwoven fabric composed ofshort-cut nanofibers obtained by cutting nanofibers (for example, seePatent Document 4). However, this has placed emphasis on achievement ofinitial efficiency, and has not been still sufficient in terms of filterlifetime.

PRIOR-ART DOCUMENTS Patent Documents

[Patent Document 1] JP-A-2004-301121

[Patent Document 2] JP-A-2006-289209

[Patent Document 3] JP-A-2007-170224

[Patent Document 4] Pamphlet of International Publication No.2008/130019

OUTLINE OF THE INVENTION Problems that the Invention is to Solve

The present invention has been made in view of the above-mentionedbackground, and an object thereof is to provide a multilayer filtermedium for use in a filter, which makes it possible to obtain a filterhaving high collection efficiency, low pressure loss and a long filterlifetime, and a filter using the multilayer filter medium for use in afilter. Means for Solving the Problems

In order to solve the above-mentioned problems, the present inventorshave made intensive studies. As a result, it has been found that abalance of pressure loss, collection efficiency and lifetime can beobtained by integrating a wet type nonwoven fabric containing short-cutnanofibers having a specific fiber diameter and fiber length(hereinafter also referred to as “ultrafine fibers”) and core-sheathconjugate type binder fibers (hereinafter also referred to as “binderfibers”) with a nonwoven fabric having a lower density than that of thewet type nonwoven fabric by lamination. The inventors have made furtherintensive studies, thereby leading to completion of the presentinvention.

Thus, according to the present invention, there is provided “amultilayer filter medium for use in a filter, which is used as aconstituent member of a filter and has a multilayer structure,comprising a wet type nonwoven fabric layer A containing 0.5 to 20% byweight, based on layer weight, of short-cut nanofibers which arecomposed of a fiber-forming thermoplastic polymer, have a single fiberdiameter (D) of 100 to 1,000 nm and are cut so that the ratio (L/D) ofthe length (L) to the single fiber diameter (D) is within the range of100 to 2,500 and core-sheath conjugate type binder fibers having asingle fiber diameter of 5 μm or more, and a nonwoven fabric layer Bhaving a lower density than that of the wet type nonwoven fabric layerA.”

In that case, the above-mentioned short-cut nanofibers are preferablyones obtained by dissolving and removing a sea component from aconjugate fiber comprising an island component composed of afiber-forming thermoplastic polymer and having an island diameter (D) of100 to 1,000 nm and a sea component composed of a polymer more easilysoluble in an alkaline aqueous solution than the above-mentionedfiber-forming thermoplastic polymer.

Further, in the above-mentioned conjugate fiber, the sea component ispreferably polyethylene terephthalate co-polymerized with 6 to 12% bymole of 5-sodium sulfonic acid and 3 to 10% by weight of polyethyleneglycol having a molecular weight of 4,000 to 12,000.

Furthermore, in the above-mentioned conjugate fiber, the islandcomponent is preferably a polyester.

In addition, in the above-mentioned conjugate fiber, the number ofislands is preferably 100 or more.

In the multilayer filter medium for use in a filter of the presentinvention, it is preferred that the nonwoven fabric layer B comprises anair-laid nonwoven fabric composed of fibers having a fiber length of 10mm or less.

Further, the nonwoven fabric layer B is preferably one containing atleast 30% by weight or more of binder fibers.

Furthermore, the ratio MB/MA of the density (MB) of the nonwoven fabriclayer B and the density (MA) of the wet type nonwoven fabric layer A ispreferably within the range of 0.1 to 0.8.

In addition, the weight ratio of the wet type nonwoven fabric layer Aand the nonwoven fabric layer B is preferably within the range of 90/10to 10/90.

Further, it is preferred that the nonwoven fabric layer B is composed oftwo or more layers.

Then, according to the present invention, there is provided a filterusing the above-mentioned multilayer filter medium for use in a filterand having the nonwoven fabric layer B arranged on a fluid inlet side.In that case, the filter is preferably an air filter for an internalcombustion engine.

ADVANTAGES OF THE INVENTION

According to the present invention, there are obtained a multilayerfilter medium for use in a filter, which makes it possible to obtain afilter having high collection efficiency, low pressure loss and a longfilter lifetime, and a filter using the multilayer filter medium for usein a filter.

MODE FOR CARRYING OUT THE INVENTION

A mode for carrying out the invention will be described in detail below.

<Wet Type Nonwoven Fabric Layer A>

A wet type nonwoven fabric layer A constituting the multilayer filtermedium for use in a filter of the present invention comprises short-cutnanofibers and core-sheath conjugate type binder fibers.

Short-Cut Nanofibers:

In the present invention, it is of vital importance that the short-cutnanofibers are composed of a fiber-forming thermoplastic polymer, have afiber diameter (D) of 100 to 1,000 nm, preferably 300 to 800 nm andparticularly preferably 550 to 800 nm, and are cut so that the ratio(L/D) of the fiber length (L) to the fiber diameter (D) is within therange of 100 to 2, 500, preferably 300 to 1, 500 and particularlypreferably 500 to 1,000. When the above-mentioned fiber diameter (D) ismore than 1,000 nm, the pore diameter of pores which appear on a surfaceof the wet type nonwoven fabric becomes uneven (that is to say, theratio of the average pore size and the maximum pore size increases).This is therefore unfavorable. On the other hand, when theabove-mentioned fiber diameter (D) is less than 100 nm, the fibersunfavorably become liable to drop out of a net at the time ofpapermaking. Further, when the above-mentioned radio (L/D) is more than2,500, entanglement of the fibers occurs at the time of papermaking tocause poor dispersion. Accordingly, the pore diameter of pores whichappear on the surface of the wet type nonwoven fabric becomes uneven(that is to say, the ratio of the average pore size and the maximum poresize increases). This is therefore unfavorable. On the other hand, whenthe above-mentioned ratio is less than 100, linkage between the fibersis extremely weakened, and in a papermaking process, transfer thereoffrom a wire part to a blanket becomes difficult, unfavorably resultingin a deterioration of process stability.

Although a method for producing the ultrafine fibers as described aboveis not particularly limited, a method disclosed in a pamphlet ofInternational Publication No. 2005/095686 is preferred. That is to say,in terms of the fiber diameter and its uniformity, preferred are onesobtained by cutting a conjugate fiber comprising an island componentcomposed of a fiber-forming thermoplastic polymer and having an islanddiameter (D) of 100 to 1,000 nm and a sea component composed of apolymer (hereinafter also referred to as an “easily soluble polymer”)which is more easily soluble in an alkaline aqueous solution than theabove-mentioned fiber-forming thermoplastic polymer, followed bysubjection to alkali reduction processing, and dissolving and removingthe above-mentioned sea component. Incidentally, the above-mentionedisland diameter can be measured by taking a photograph of across-section of the fiber using a transmission type electronmicroscope. In addition, when the island has a noncircular cross-sectionshape other than a circular cross-section shape, the diameter of acircumscribed circle thereof is used as the above-mentioned islanddiameter (D).

Here, when the dissolution rate ratio of the polymer which is easilysoluble in an alkaline aqueous solution and forms the sea component tothe fiber-forming thermoplastic polymer which forms the island componentis 200 or more and preferably from 300 to 3,000, island separability isimproved. This is therefore preferred. When the dissolution rate ratiois less than 200 times, the separated island component in a surfacelayer portion of a fiber cross-section is dissolved because of the smallfiber diameter, while the sea component in the center portion of thefiber cross-section is dissolved. Accordingly, the sea component in thecenter portion of the fiber cross-section cannot be completely dissolvedand removed, despite the loss of the sea-corresponding weight, whichleads to thickness unevenness of the island component or solvent erosionof the island component itself, and the ultrafine short fibers having auniform fiber diameter might not be obtained.

Preferred examples of the easily soluble polymers which form the seacomponent include particularly polyesters, which have good fiber-formingproperties, aliphatic polyamides and polyolefins such as polyethyleneand polystyrene. Further specific examples of the polymers easilysoluble in an alkaline aqueous solution optimally includepolyester-based polymers such as polylactic acid, ultrahigh molecularweight polyalkylene oxide condensation polymers and copolymerizedpolyesters of polyalkylene glycol-based compounds and 5-sodiumsulfoisophthalic acid. The alkaline aqueous solution used herein meansan aqueous solution of potassium hydroxide, sodium hydroxide or thelike. In addition to this, examples thereof include formic acid toaliphatic polyamides such as nylon 6 and nylon 66, trichloroethylene topolystyrene, hydro-carbon-based solvents such as hot toluene and xyleneto polyethylene (particularly, high-pressure low-density polyethylene orlinear low-density polyethylene) and hot water to polyvinyl alcohol orethylene-modified vinyl alcohol-based polymers.

Of the polyester-based polymers, preferred is a polyethyleneterephthalate-based copolymerized polyester having an intrinsicviscosity of 0.4 to 0.6, which is copolymerized with 6 to 12% by mole of5-sodium sulfoisophthalic acid and 3 to 10% by weight of polyethyleneglycol having a molecular weight of 4,000 to 12,000. Here, 5-sodiumsulfoisophthalic acid contributes to hydrophilicity and improvement inmelt viscosity, and polyethylene glycol (PEG) improves hydrophilicity.Further, PEG having a higher molecular weight has a morehydrophilicity-increasing action which is considered to be caused by itshigher-order structure. However, reactivity thereof is deteriorated toform a blend system, so that problems might be raised with regard toheat resistance and spinning stability. Furthermore, when thecopolymerized amount of PEG exceeds 10% by weight, it causes a meltviscosity-decreasing action. This is therefore unfavorable.

On the other hand, suitable examples of the slightly soluble polymersforming the island component include polyamides, polyesters, polyolefinsand the like. Specifically, in use requiring mechanical strength or heatresistance, preferred as the polyesters are polyethylene terephthalate(hereinafter also referred to as “PET”), polytrimethylene terephthalate,polybutylene terephthalate, polyethylene naphthalate and copolymershaving these as main repeating units, which are copolymerized witharomatic dicarboxylic acids such as isophthalic acid and metal salts of5-sulfoisphthalic acid, aliphatic dicarboxylic acids such as adipic acidand sebacic acid, hydroxycarboxylic acid condensates such asε-caprolactone, glycol components such as diethylene glycol,trimethylene glycol, tetramethylene glycol and hexamethylene glycol, orthe like. Further, as the polyamides, preferred are aliphatic polyamidessuch as nylon 6 and nylon 66. On the other hand, the polyolefins arecharacterized by that they are hard to be attacked by acids or alkalis,that they can be used as a binder component after being taken out as theultrafine fibers because of their relatively low melting point, and thelike. Preferred examples thereof include high-density polyethylene,medium-density polyethylene, high-pressure low-density polyethylene,linear low-density polyethylene, isotactic polypropylene,ethylene-propylene copolymers, ethylene copolymers of vinyl monomerssuch as maleic anhydride, and the like.

Further, the island component may have not only a circularcross-section, but also a noncircular cross-section. In particular,aliaromatic polyesters such as polyethylene terephthalate,polytrimethylene terephthalate, polybutylene terephthalate, polyethyleneterephthalate isophthalate having an isophthalic acid copolymerizationratio of 20% by mole or less and polyethylene naphthalate or aliphaticpolyamides such as nylon 6 and nylon 66 can be preferably applied to userequiring heat resistance and strength, compared to ultrafinefibrillated fibers obtained from polyvinyl alcohol/polyacrylonitrileblend spinning fibers, because they have heat resistance due to theirhigh melting point and mechanical characteristics.

Incidentally, the polymers forming the sea component and the polymersforming the island component may contain various additives such asorganic fillers, antioxidants, heat stabilizers, light stabilizers,flame retardants, lubricants, antistatic agents, corrosion inhibitors,crosslinking agents, foaming agents, fluorescent agents, surfacesmoothing agents, surface gloss improvers and release improvers such asfluororesins, as needed, within the range not exerting influence onfiber-forming properties and physical properties of the ultrafine fibersafter extraction.

In the above-mentioned sea-island type conjugate fiber, it is preferredthat the melt viscosity of the sea component at the time of meltspinning is higher than the melt viscosity of the island componentpolymer. In the case where such a relationship is satisfied, even whenthe conjugate weight ratio of the sea component becomes as low as lessthan 40%, islands are less likely to stick together, or almost all ofthe islands are less likely to stick, resulting in easy formation of thesea-island type conjugate fiber.

The preferred melt viscosity ratio (sea/island) is within the range of1.1 to 2.0, particularly 1.1 to 1.5. When this ratio is less than 1.1,the islands become liable to stick together at the time of meltspinning. On the other hand, in the case of exceeding 2.0, the spinningbehavior tends to be deteriorated, because the difference in viscosityis too large.

Then, the number of islands is preferably 100 or more (more preferably,from 300 to 1,000). Further, the sea-island conjugate weight ratio (sea:island) thereof is preferably within the range of 5:95 to 95:5. Withinsuch a range, the thickness of the sea component between the islands canbe decreased to make it easy to dissolve and remove the sea component,resulting in easy conversion of the islands to the ultrafine fibers.This is therefore preferred. Here, when the ratio of the sea componentexceeds 95%, the thickness of the sea component becomes too thick. Onthe other hand, in the case of less than 5%, the amount of the seacomponent becomes too small, resulting in easy occurrence of stickingbetween the islands.

As a spinneret used for melt spinning, there can be used any one havinga hollow pin group or a fine orifice group for forming the islandcomponents. For example, there may be used a spinning spinneret in whichthe island components extruded from the hollow pins or the fine orificesand a sea component flow extruded from a flow passage which is designedin such a form as to fill a gap therebetween are allowed to meettogether, followed by compression thereof, thereby forming a sea-islandcross-section. The sea-island type conjugate fiber extruded issolidified by a cooling air and taken up by a rotary roller or anejector which is set to a predetermined take-up speed to obtain anundrawn yarn. Although not particularly limited, this take-up speed isdesirably from 200 to 5,000 m/min. Less than 200 m/min results in poorproductivity, whereas exceeding 5,000 m/min results in poor spinningstability.

The undrawn yarn obtained may be subjected to a cut process or asubsequent extraction process as such depending on use and purpose ofthe ultrafine fibers obtained after extraction of the sea component, orin order to match intended strength, elongation and thermal shrinkagecharacteristics, can be subjected to the cut process or the subsequentextraction process through a drawing process or a heat treatmentprocess. The drawing process may be a separate draw system in whichspinning and drawing are performed in separate steps or a direct drawsystem in which drawing is performed immediately after spinning in onestep.

Then, such a conjugate fiber is cut so that the ratio (L/D) of thelength (L) to the single fiber diameter (D) is within the range of 100to 2,500. Such cutting is preferably performed by cutting the undrawn ordrawn yarn as such or a tow bundled by tens to millions of yarns, by aguillotine cutter, a rotary cutter or the like. Further, cutting may beperformed in a process after the following extraction process (alkalireduction processing).

In the above-mentioned extraction process (alkali reduction processing),the ratio (bath ratio) of the fibers and the alkaline solution ispreferably from 0.1 to 5%, and more preferably from 0.4 to 3%. When itis less than 0.1%, process properties such as water discharge mightbecome difficult, although the fibers much come into contact with thealkaline solution. On the other hand, when it exceeds 5%, entanglementof the fibers might occur at the time of the alkali reductionprocessing, because the fiber amount is too large. Incidentally, thebath ratio is defined by the following equation:

Bath ratio (%)=[fiber weight (gr)/alkaline aqueous solution weight(gr)]×100

Further, the processing time of the alkali reduction processing ispreferably from 5 to 60 minutes, and more preferably from 10 to 30minutes. When it is less than 5 minutes, the alkali reduction processingmight become insufficient. On the other hand, when it exceeds 60minutes, the island component might also be reduced in weight.

Incidentally, the processing temperature at the time of the alkalireduction processing is usually from 50 to 90° C., and preferably fromabout 60 to 80° C.

Further, alkalis used for the alkali reduction processing include sodiumhydroxide and the like.

Further, in the alkali reduction processing, the alkali concentration ispreferably from 2 to 10%. When it is less than 2%, the alkali isdeficient to cause a possibility that the rate of caustic reduction isextremely reduced because of alkali deficiency. On the other hand, whenit exceeds 10%, the caustic reduction excessively proceeds to cause apossibility that the island component is also reduced in weight.

Methods for caustic reduction include a method of putting a conjugatefiber cut (or not cut) in an alkaline solution, treating it underpredetermined conditions for a predetermined period of time, thereafter,putting it in water again, once through a dehydration process, allowingneutralization and dilution to proceed using an organic acid such asacetic acid or oxalic acid, and finally performing dehydration, or amethod of previously performing the neutralization treatment after thetreatment for a predetermined period of time, further pouring water toallow the dilution to proceed, and thereafter performing thedehydration. In the former, production (processing) in small quantitiescan be performed because of batch type treatment. On the other hand, theneutralization treatment requires time, so that productivity is somewhatlow. In the latter, semicontinuous production is possible, but there aredisadvantages that the acid aqueous solution is required in largeamounts at the time of the neutralization treatment and that water isrequired in large amounts for the dilution.

A treatment equipment is not limited in any way. However, from theviewpoint of preventing fiber dropout at the time of the dehydration, amesh-like material (for example, an alkali non-hydrolyzable bag or thelike) having an aperture ratio (which means the area of opening portionsper unit area) of 10 to 50% as disclosed in Japanese Patent No. 3678511is preferably applied. When the above-mentioned aperture ratio is lessthan 10%, the passing through of water is extremely slow. On the otherhand, when it exceeds 50%, fiber dropout might occur.

Furthermore, in order to increase dispersibility, after the alkalireduction processing, a dispersing agent (for example, type YM-81manufactured by Takamatsu Oil & Fat Co., Ltd.) is preferably allowed toadhere onto fiber surfaces in an amount of 0.1 to 5.0% by weight basedon the fiber weight.

In the above-mentioned wet type nonwoven fabric layer A, the ratio ofthe short-cut nanofibers in the wet type non-woven fabric layer A isfrom 0.5 to 20% by weight, preferably from 2 to 20% by weight, and morepreferably from 3 to 10% by weight. When it is less than 0.5% by weight,not only satisfactory collection efficiency cannot be obtained, but alsotexture unevenness as the nonwoven fabric might occur. This is thereforeunfavorable. On the other hand, when it exceeds 20% by weight, thenonwoven fabric becomes too dense, so that water filtering properties ina papermaking process is extremely deteriorated to cause a reduction inproductivity or an excessive increase in pressure loss. This istherefore unfavorable.

Core-Sheath Conjugate Type Binder Fibers:

In the wet type nonwoven fabric layer A used in the present invention,the structure thereof is maintained by adhesion of the core-sheathcomposition type binder fibers having a single fiber diameter of 5 μm ormore, preferably 5 to 20 μm and more preferably 7 to 15 μm. Here, whenthe single fiber diameter of the core-sheath composition type binderfibers is less than 5 μm, rigidity of the fibers themselves isunfavorably decreased to make it difficult to maintain the structure ofthe wet type nonwoven fabric layer A. On the other hand, when it exceeds20 μm, the number of constituent binder fibers in the wet type nonwovenfabric is decreased to decrease their adhesion points, which might causea decrease in rigidity.

Further, the core-sheath composition type binder fibers are preferablycut to a fiber length of 3 to 100 mm.

As such core-sheath conjugate type binder fibers, it is preferred that apolymer having a melting point at least 40° C. lower than that of thepolymer which forms the above-mentioned short-cut nanofibers is arrangedon surfaces thereof as a thermal adhesive component.

The polymers arranged as the thermal adhesive component herein includepolyurethane-based elastomers, polyester-based elastomers, non-elasticpolyester-based polymers and copolymers thereof, polyolefin-basedpolymers and copolymers thereof, polyvinyl alcohol-based polymers andthe like.

Of these, the polyurethane-based elastomers are polymers obtained byreaction of low-melting polyols having a molecular weight of about 500to 6,000, such as dihydroxy polyethers, dihydroxy polyesters, dihydroxypolycarbonates and dihydroxy polyesteramides, organic diisocyanateshaving a molecular weight of 500 or less, such as p,p′-diphenylmethanediisocyanate, tolylene diisocyanate, isophorone diisocyanate,hydrogenated diphenylmethane diisocyanate, xylylene diisocyanate,2,6-diisocyanate methyl caproate and hexamethylene diisocyanate, andchain extenders having a molecular weight of 500 or less, such asglycols, aminoalcohols and triols.

Of these polymers, particularly preferred is a polyurethane usingpolytetramethylene glycol as the polyol, or poly-ε-caprolactam orpolybutylene adipate. The organic diisocyanates in this case includep,p′-bishydroxyethoxy-benzene and 1,4-butanediol.

Further, the polyester-based elastomers include polyetherestercopolymers obtained by copolymerizing thermoplastic polyesters as hardsegments and poly(alkylene oxide) glycols as soft segments, morespecifically, terpolymers composed of at least one dicarboxylic acidselected from alicyclic dicarboxylic acids such as terephthalic acid,iso-phthalic acid, naphthalene-2,6-dicarboxylic acid,naph-thalene-2,7-dicarboxylic acid, diphenyl-4,4′-dicarboxylic acid and1,4-cyclohexanedicarboxylic acid, aliphatic di-carboxylic acids such assuccinic acid, oxalic acid, adipic acid, sebacic acid, dodecanedioicacid and dimer acid, or ester-forming derivatives thereof; at least oneof diol components selected from aliphatic dials such as 1,4-butanediol,ethylene glycol, trimethylene glycol, tetramethylene glycol,pentamethylene glycol, hexamethylene glycol, neopentyl glycol anddecamethylene glycol, alicyclic diols such as1,1-cyclo-hexanedimethanol, 1,4-cyclohexanedimethanol andtricyclo-decanedimethanol, ester-forming derivatives thereof or thelike; and at least one of poly(alkylene oxide) glycols having an averagemolecular weight of about 400 to 5,000, such as poly(ethylene glycol),poly(1,2- and 1,3-propylene oxide) glycol, poly(tetramethylene oxide)glycol, a copolymer of ethylene oxide and propylene oxide and acopolymer of ethylene oxide and tetrahydrofuran.

In particular, in view of adhesiveness, thermal characteristics andstrength, preferred is a block co-polymerized polyetherester obtained byusing poly-butylene-based terephthalate as a hard segment andpoly-oxybutylene glycol as a soft segment. In this case, the poly-esterportion constituting the hard segment is a polybutylene terephthalate inwhich a main acid component is terephthalic acid, and a main diolcomponent is a butylene glycol component. Of course, part of the acidcomponent (usually, 30% by mole or less) may be replaced with anotherdicarboxylic acid component or another oxycarboxylic acid component.Similarly, part of the glycol component (usually, 30% by mole or less)may be replaced with a dioxy component other than the butylene glycolcomponent. Further, the polyether portion constituting the soft segmentmay also be a polyether replaced with a dioxy component other thanbutylene glycol.

The copolymerized polyester-based polymers include copolymerized esterscontaining aliphatic dicarboxylic acids such as adipic acid and sebacicacid, aromatic dicarboxylic acids such as phthalic acid, isophthalicacid and naphthalene-dicarboxylic acid and/or alicyclic dicarboxylicacids such as hexahydroterephthalic acid and hexahydroisophthalic acid,and aliphatic or alicyclic diols such as diethylene glycol, polyethyleneglycol, propylene glycol and paraxylene glycol, in the predeterminednumbers, to which oxy acids such as parahydroxybenzoic acid are added asneeded. For example, there can be used a polyester in which isophthalicacid and 1,6-hexandiol are added to and copolymerized with terephthalicacid and ethylene glycol.

Further, the polyolefin-based polymers include, for example, low-densitypolyethylene, high-density polyethylene, polypropylene and further,modified products thereof.

In particular, as the core-sheath conjugate type binder fibers, it ismore preferred that a polyester is arranged as a core and that alow-melting polyester is arranged as a sheath, from the viewpoints ofadhesiveness to the short-cut nanofibers and processing properties (suchas dispersibility) in the papermaking process.

Incidentally, various stabilizers, UV-absorbers, thickening branchingagents, delusterants, colorants and other various improvers may beblended in the above-mentioned polymers as needed.

In the core-sheath conjugate type binder fibers, it is more preferredthat the polyester is arranged as the core and that the low-meltingpolyester is arranged as the sheath, from the viewpoints of adhesivenessto the short-cut nanofibers and processing properties (such asdispersibility) in the papermaking process, as described above. Here,the thermal adhesive component preferably occupies at least a half ofthe surface area. The weight ratio of the thermal adhesive component andthe other side component is suitably within the range of 10/90 to 70/30in the conjugate ratio (weight ratio). The morphology of the core-sheathconjugate type binder fibers is a core-sheath type. In this core-sheathtype core-sheath conjugate type binder fiber, the thermal adhesivecomponent forms the sheath portion, and the other side component formsthe core portion. This core portion may be arranged eitherconcentrically or eccentrically.

Other Fibers:

Incidentally, in the wet type nonwoven fabric layer A, various syntheticfibers (polyethylene terephthalate, polytrimethylene terephthalate,nylon, olefinic series and aramid series), natural pulp such as woodpulp or linter pulp, synthetic pulp mainly composed of aramid orpolyethylene, and the like can be used as the other fibers other thanthe above-mentioned short-cut nanofibers and binder fibers. Inparticular, drawn polyethylene terephthalate short fibers composed ofpolyethylene terephthalate and having a single fiber diameter of 2 to 30μm and a fiber length of 3 to 10 mm are preferred from the viewpoint ofdimensional stability and the like.

The ratio of the above-mentioned other fibers is 80% by weight or less,and more preferably about 60 to 80% by weight, in the wet type nonwovenfabric layer A.

When the multilayer filter medium for use in a filter of the presentinvention is used as a filter, the above-mentioned wet type nonwovenfabric layer A is preferably disposed on a fluid outflow side (cleanside). When the above-mentioned wet type nonwoven fabric layer A isdisposed on the fluid outflow side (clean side), it plays a role incollecting fine dusts. Accordingly, the above-mentioned wet typenonwoven fabric layer A becomes the densest structure in the entirefilter medium. However, when it is excessively dense to become too lowin air permeation, it becomes difficult to achieve low pressure loss, ahigh lifetime and the like. It is therefore important to forma structurewhich secures a certain air permeability. Such an air permeability ispreferably 10 cc/cm²/s or more, and more preferably 20 cc/cm²/s or more.

Incidentally, the basis weight of the wet type nonwoven fabric layer Ais usually from 30 to 150 g/m², preferably from 50 to 120 g/m², andparticularly preferably from about 80 to 100 g/m².

Method for Producing Wet Type Nonwoven Fabric Layer A:

Then, as a method for producing the wet type nonwoven fabric layer A ofthe present invention, preferred is a production method in whichpapermaking is performed with an ordinary fourdrinier machine, short netpapermaking machine or cylinder paper machine, or as multiplayerpapermaking combining a plurality of these machines, followed by heattreatment. In that case, as the heat treatment process, either a Yankeedryer or an air-through dryer may be used after the papermaking process.Further, calendaring or embossing may be performed as needed.

<Nonwoven Fabric Layer B>

In the present invention, it is of vital importance that the density ofthe nonwoven fabric layer B is lower than that of the above-mentionedwet type nonwoven fabric layer A. In particular, it is preferred thatthe ratio (MB/MA) of the density (MB) of the nonwoven fabric layer B andthe density (MA) of the wet type nonwoven fabric layer A is within therange of 0.1 to 0.8. More preferably, it is from 0.1 to 0.6, andparticularly preferably from 0.12 to 0.5.

When the multilayer filter medium for use in a filter of the presentinvention is used as a filter, the nonwoven fabric layer B is preferablydisposed on a fluid inflow side (dust side). The nonwoven fabric layer Bis disposed on the fluid inflow side (dust side), whereby it plays arole in collecting relatively large dusts. In the case where the ratio(MB/MA) exceeds 0.8, for example, when the density of the nonwovenfabric layer B is equal to or higher than that of the above-mentionedwet type nonwoven fabric layer A, the nonwoven fabric layer B collectsnot only the relatively large dusts, but also small dusts. Accordingly,the dusts are deposited on a surface thereof to cause clogging,resulting in not only a failure to effectively utilize the wet typenonwoven fabric layer A, but also a decrease in lifetime as the filter.This is therefore unfavorable. On the other hand, when the ratio (MB/MA)is less than 0.1, the dusts are scarcely collected by the nonwovenfabric layer B to pass through it. Accordingly, a burden on the wet typenonwoven fabric layer A is increased to unfavorably cause a decrease indust retaining amount (lifetime).

Further, the nonwoven fabric layer B is more preferably a dry typenonwoven fabric, when the balance of density, basic strength and thelike is considered. As a method for producing the above-mentionednonwoven fabric layer B, there is applicable needle punching, waterneedling, thermal bonding or point bonding, in which the nonwoven fabricis produced through an ordinary carding process, or an airlaid processin which the short-cut fibers are opened in the air to obtain a web.When the texture, density or the like of the nonwoven fabric isconsidered, the airlaid process is more preferred. This nonwoven fabriclayer B is not only a single layer, but also may be a multilayer(different in configuration, or the like) obtained by lamination oflayers produced by the same production method or by different productionmethods, without any problem. Further, the nonwoven fabric layer B hasno problem even when produced by a production method other than that ofthe dry type nonwoven fabric.

There is no particular limitation on the fibers used in the nonwovenfabric layer B, but in order to obtain rigidity as the filter, it ispreferred that the binder fibers are contained in an amount of at least30% by weight, and it is more preferred that the binder fibers are thecore-sheath type binder fibers as described above.

That is to say, of the binder fibers used in such a nonwoven fabriclayer B, the core-sheath type binder fibers have a single fiber diameterof 5 μm or more, preferably 5 to 20 μm and more preferably 7 to 15 μm,and are preferably cut to a fiber length of 3 to 100 mm.

The other constitutions and functions of the core-sheath type binderfibers are the same as those of the core-sheath type binder fibers usedin the nonwoven fabric layer A, so that explanation thereof is omitted.

Incidentally, when the nonwoven fabric layer B is produced by theairlaid process, it is preferred that the fibers have a fiber length of10 mm or less.

Further, the above-mentioned nonwoven fabric layer B preferably has ahigh air permeability, and the air permeability thereof is preferably 50cc/cm²/s or more, and further, more preferably 100 cc/cm²/s or more.

Incidentally, the basis weight of the nonwoven fabric layer B is usuallyfrom 20 to 100 g/m², preferably from 30 to 80 g/m², and particularlypreferably from about 40 to 60 g/m².

<Multilayer Filter Medium for Use in Filter>

In the multilayer filter medium for use in a filter of the presentinvention, the weight ratio of the wet type nonwoven fabric layer A andthe nonwoven fabric layer B is preferably within the range of 90/10 to10/90. Further, it is preferred that the nonwoven fabric layer B iscomposed of two or more layers.

A method for integrating the wet type nonwoven fabric layer A with thenonwoven fabric layer B is not particularly limited. Both nonwovenfabric layers have the binder fibers, so that it is not necessarilyessential to require a new binder resin at an interface between bothnonwoven fabric layers, and it becomes possible to perform integrationby passing through an air-through dryer or a process such as embossingor point bonding.

Specific examples of the methods for integrating the wet type nonwovenfabric layer A with the nonwoven fabric layer B include a method ofperforming integration by laminating the wet type nonwoven fabric layerA and the nonwoven fabric layer B in a hot-air suction type dryingfurnace elevated to a temperature of 140 to 160° C., and setting aclearance in such a manner that compression is performed by 5% based onthe thickness in a laminated state.

Incidentally, in the above-mentioned multilayer filter medium for use ina filter, an attachment other than the above-mentioned wet type nonwovenfabric layer A and nonwoven fabric layer B, for example, a sheet-likestructure such as a woven fabric or a nonwoven fabric having a coarsestructure (air permeability: 100 cc/cm²/s or more), which does not exertan influence on filter performance, may be laminated thereon to improverigidity. Further, also the shape of the multilayer filter medium foruse in a filter is not limited to a tabular form, and may be any shape.Furthermore, known functional processing such as ordinary waterrepellent finish, fire proofing, flame retarding, dying and minus iongeneration processing may be added.

<Filter>

Then, the filter of the present invention is a filter which is obtainedusing the above-mentioned multilayer filter medium for use in a filterand in which the nonwoven fabric layer B is disposed on a fluid inflowside. In such a filter, relatively large dusts are collected by thenonwoven fabric layer B disposed on the fluid inflow side (dust side),and fine dusts are collected by the wet type nonwoven fabric layer Adisposed on the fluid outflow side (clean side). As a result, highcollection efficiency, low pressure loss and a long filter lifetime areobtained.

The filter of the present invention has high collection efficiency, lowpressure loss and a long filter lifetime, so that it can be suitablyused as an air filter for an internal combustion engine such as anintake air filter for an internal combustion engine. However, it mayalso be used as a filter for other use.

The total basis weight of such a filter is usually form 60 to 200 g/m²,preferably from 80 to 180 g/m², and particularly preferably from about100 to 150 g/m².

Further, the air permeability of the filter obtained by combining thenonwoven fabric layer B and the wet type nonwoven fabric layer A isusually from 20 to 150 cc/cm²/s and preferably from about 50 to 80cc/cm²/s.

EXAMPLES

Examples and comparative examples of the present invention will bedescribed in detail below, but the present invention should not beconstrued as being limited thereby. Incidentally, respective measurementitems in examples were measured by the following methods.

(1) Melt Viscosity

A polymer after drying treatment was set to an orifice whose temperaturehad been set to the melting temperature of an extruder at the time ofspinning, melted and held for 5 minutes, and then, extruded by applyingseveral levels of load. The shear rate and the melt viscosity at thattime were plotted. The plotted points were smoothly connected to preparea shear rate-melt viscosity curve, and the melt viscosity at the timewhen the shear rate was 1,000 sec⁻¹ was measured.

(2) Measurement of Island Diameter

A fiber cross-sectional photograph was taken at ×30,000 magnificationunder a transmission type electron microscope TEM, and measurement wasperformed. The measurement was performed utilizing the lengthmeasurement capabilities possessed by the TEM.

Further, in the absence of the TEM, the photograph taken may be enlargedwith a copier and measured with a ruler in view of a reduction scale.However, an average value (n=20) of major axes and minor axes in fibercross-sections was used as the fiber diameter.

(3) Fiber Length

In a state where an ultrafine short fiber before dissolution and removalof a sea component was laid on a base plate, the fiber length thereofwas measured at ×20 to ×500 magnification under a scanning electromicroscope (SEM). The measurement was performed utilizing the lengthmeasurement function of the SEM.

(4) Basis Weight

Measurement was performed on the basis of JIS P8124 (Measuring Method ofBasis Weight in GSM of Paper).

(5) Thickness

Measurement was performed on the basis of JIS P8118 (Testing Method ofThickness and Density of Paper and Paper Board).

(6) Density

Measurement was performed on the basis of JIS P8118 (Testing Method ofThickness and Density of Paper and Paper Board).

(7) Collection Efficiency

When the flow rate at the time of sample passing was 16.7 cm/sec and thedust concentration was 1 g/m³, using ISO FINE dust, the transmittance ofthe dust before and after the sample passing through was taken as thecollection efficiency.

(8) Pressure Loss

The pressure loss was determined at the time of performing theabove-mentioned collection efficiency (flow rate: 16.7 cm/sec).

(9) Filter Lifetime (DHC)

The above-mentioned collection efficiency test was performed, and thedust retaining amount (weight increase) at the time when an increase inpressure loss reached 2 kPa was taken as the DHC.

(10) Air Permeability

Measurement was performed on the on the basis of JIS L1096 (TestingMethod of General Woven Fabric).

Example 1

Using polyethylene terephthalate having a melt viscosity of 120 Pa·secat 285° C. as an island component and modified polyethyleneterephthalate having a melt viscosity of 135 Pa·sec at 285° C., whichwas obtained by copolymerizing 4% by weight of polyethylene glycolhaving an average molecular weight of 4,000 and 9% by mole of 5-sodiumsulfoisophthalic acid, as a sea component, spinning was performed at aweight ratio of sea:island=10:90 using a spinneret having an islandnumber of 400, and taken up at a spinning speed of 1,500 m/min. Thedifference in alkali reduction rate was 1,000 times. This was drawn to3.9 times, and cut to 1,000 μm with a guillotine cutter to obtain anultrafine short fiber precursor. This was subjected to alkali reductionwith a 4% NaOH aqueous solution at 75° C. to reduce the weight by 10%.Asa result, it was confirmed that ultrafine short fibers having arelatively uniform fiber diameter and fiber length were formed. Theresulting fibers were used as short-cut nanofibers (750 nm, 0.8 mm,L/D=1,067).

On the other hand, core-sheath conjugate type binder short fibers(fineness: 1.1 dtex, single fiber diameter: 10 μm, fiber length: 5 mm,no crimp, core/sheath=50/50, core:polyethylene terephthalate having amelting point of 256° C., sheath: copolymerized polyester having asoftening point of 110° C., which was mainly composed of terephthalicacid, isophthalic acid, ethylene glycol and diethylene glycol) as binderfibers, and, in addition thereto, polyethylene terephthalate shortfibers (fineness: 1.7 dtex, single fiber diameter: 12 μm, fiber length:5 mm, no crimp) were mixed with the short-cut nanofibers at apredetermined weight ratio (short-cut nanofibers/binder fibers/otherfibers=5/50/45), followed by stirring. The resulting mixture wassubjected to papermaking to 91 g/m² by TAPPI (a square type sheetmachine manufactured by Kumagai Riki Kogyo Co., Ltd., hereinafter thesame), followed by drying with a Yankee dryer (120° C.×2 minutes) toobtain a sheet (wet type nonwoven fabric layer A) . Incidentally, thiswet type nonwoven fabric layer A had an air permeability of 81 cc/cm²/s.

On the other hand, as a nonwoven fabric layer B, using core-sheathconjugate type binder short fibers (fineness: 1.7 dtex, single fiberdiameter: 12 μm, fiber length: 5 mm, crimped, core/sheath=50/50, core:polyethylene terephthalate having a melting point of 256° C., sheath:copolymerized polyester having a softening point of 110° C., which wasmainly composed of terephthalic acid, isophthalic acid, ethylene glycoland diethylene glycol) , a web having a basis weight of 39 g/m² wasformed with an airlaid test production apparatus (Japanese Patent No.3880456), and then, heat treated at 140° C.×5 min with a hot-air suctiontype dryer to obtain an airlaid nonwoven fabric sheet (nonwoven fabriclayer B). Incidentally, this nonwoven fabric layer B had an airpermeability of 210 cc/cm²/s.

Both were laminated on each other, and re-heat treated at 150° C.,thereby remelting the core-sheath conjugate type binder short fiberscontained in the respective layers to obtain an integrated sheet. At thetime of measuring the filter performance, evaluation of the basicperformance was performed with the low-density layer (nonwoven fabriclayer B) disposed on a fluid inflow side (dust side). The resultsthereof are described in Table 1.

Example 2

A sheet was prepared in the same manner as in Example 1 with theexception that the respective basis weights of the wet type nonwovenfabric layer A and the nonwoven fabric layer B were changed (wet typenonwoven fabric layer A: from 91 to 51, nonwoven fabric layer B: from 39to 81). The results of performance evaluation of the sheet are describedin Table 1. Incidentally, this wet type nonwoven fabric layer A had anair permeability of 230 cc/cm²/s, and the nonwoven fabric layer B had anair permeability of 131 cc/cm²/s.

Example 3

A sheet was prepared in the same manner as in Example 1 with theexception that the mixing ratio in the wet type nonwoven fabric layer Awas changed to short-cut nanofibers/binder fibers/other fibers=15/50/35.The results of performance evaluation of the sheet are described inTable 1. Incidentally, this wet type nonwoven fabric layer A had an airpermeability of 61 cc/cm²/s.

Example 4

A sheet was prepared in the same manner as in Example 1 with theexception that the mixing weight ratio in the wet type nonwoven fabriclayer A was changed to short-cut nanofibers/binder fibers/otherfibers=1/50/49. The results of performance evaluation of the sheet aredescribed in Table 1. Incidentally, this wet type nonwoven fabric layerA had an air permeability of 209 cc/cm²/s.

Comparative Example 1

A sheet was prepared in the same manner as in Example 1 with theexception that the mixing weight ratio in the wet type nonwoven fabriclayer A was changed to short-cut nanofibers/binder fibers/otherfibers=25/50/25. The results of performance evaluation of the sheet aredescribed in Table 2. Incidentally, this wet type nonwoven fabric layerA had an air permeability of 9 cc/cm²/s.

Comparative Example 2

A sheet was prepared in the same manner as in Example 1 with theexception that the mixing weight ratio in the wet type nonwoven fabriclayer A was changed to short-cut nanofibers/binder fibers/otherfibers=0.1/50/49.9. The results of performance evaluation of the sheetare described in Table 2. Incidentally, this wet type nonwoven fabriclayer

A had an air permeability of 241 cc/cm²/s.

Comparative Example 3

The wet type nonwoven fabric layer A formed in Example 1 was singly usedto obtain a nonwoven fabric sheet increased in basis weight (from 91 to141). The results of performance evaluation of the sheet are describedin Table 2. Incidentally, this wet type nonwoven fabric layer A had anair permeability of 53 cc/cm²/s.

TABLE 1 Table 1 (Filter Medium for Use in Filter) Exam- Exam- Exam-Exam- Fiber Diameter × Fiber Length Unit ple 1 ple 2 ple 3 ple 4Nonwoven Short-Cut Nano- 750 nm × 0.8 mm L/D 1067 wt % 5 5 15 1 Fabricfibers Layer A Core-Sheath 1.1 dtex × 5 mm wt % 50 50 50 50 ConjugateType Binder Fibers Other Fibers 1.7 dtex × 5 mm wt % 45 45 35 49Properties Basis Weight g/m² 91 51 89 91 Thickness mm 0.45 0.23 0.370.55 Density g/cm³ 0.202 0.222 0.241 0.165 Nonwoven Core-Sheath 1.7 dtex× 5 mm wt % 100 100 100 100 Fabric Conjugate Type Layer B Binder FibersProperties Basis Weight g/m² 39 81 42 41 Thickness mm 1.5 2.6 1.5 1.5Density g/cm³ 0.026 0.031 0.028 0.027 Compos- Properties Basis Weightg/m² 130 132 131 132 ite Thickness mm 1.95 2.83 1.87 2.05 Density g/cm³0.067 0.047 0.070 0.064 Nonwoven Fabric LayerA/Nonwoven 2.3 0.6 2.1 2.2Fabric Layer B Density of Nonwoven Fabric B (MB)/ 0.13 0.14 0.12 0.17Density of Nonwoven Fabric A (MA) Filter Perform- Collection Efficiency% 99.98 99.83 99.99 99.78 ance Pressure Loss Pa 638 487 762 614 DHC g/m²362 452 261 451 Remarks *1 *2 *3 *4

TABLE 2 Table 2 (Filter Medium for Use in Filter) Com. Ex- Com. Ex- Com.Ex- Fiber Diameter × Fiber Length Unit ample 1 ample 2 ample 3 Non-Short-Cut 750 nm × 0.8 mm L/D 1067 wt % 25 0.1 5 woven Nanofibers FabricCore-Sheath 1.1 dtex × 5 mm wt % 50 50 50 Layer Conjugate Type A BinderFibers Other Fibers 1.7 dtex × 5 mm wt % 25 49.9 45 Properties BasisWeight g/m² 92 91 141 Thickness mm 0.32 0.54 0.69 Density g/cm³ 0.2880.169 0.204 Non- Core-Sheath 1.7 dtex × 5 mm wt % 100 100 — wovenConjugate Type Fabric Binder Fibers Layer Properties Basis Weight g/m²41 42 — B Thickness mm 1.5 1.5 — Density g/cm³ 0.027 0.028 — Compos-Properties Basis Weight g/m² 133 133 141 ite Thickness mm 1.82 2.04 0.69Density g/cm³ 0.073 0.065 0.204 Nonwoven Fabric Layer A/Nonwoven 2.2 2.2— Fabric Layer B Density of Nonwoven Fabric B (MB)/ 0.10 0.17 — Densityof Nonwoven Fabric A (MA) Filter Collection Efficiency % 99.99 93.199.98 Performance Pressure Loss Pa 876 563 752 DHC g/m² 87 96 85 Remarks*5 *6 *7

INDUSTRIAL APPLICABILITY

According to the present invention, there are provided a multilayerfilter medium for use in a filter, which makes it possible to obtain afilter having high collection efficiency, low pressure loss and a longfilter lifetime, and a filter using the multilayer filter medium for usein a filter. The filter is also useful as a filter for an indoor airconditioner, a cooler, a heater (electric or oil-fired), an automotiveair conditioner, an air cleaner, a clean room, an indoor humidifier orthe like, as well as an air filter for an internal combustion enginesuch as an intake air filter for an internal combustion engine. Thus,the industrial value thereof is extremely large.

1. A multilayer filter medium for use in a filter, which is used as aconstituent member of a filter and has a multilayer structure,comprising a wet type nonwoven fabric layer A containing 0.5 to 20% byweight, based on layer weight, of short-cut nanofibers which arecomposed of a fiber-forming thermoplastic polymer, have a single fiberdiameter (D) of 100 to 1,000 nm and are cut so that the ratio (L/D) ofthe length (L) to the single fiber diameter (D) is within the range of100 to 2,500 and core-sheath conjugate type binder fibers having asingle fiber diameter of 5 μm or more, and a nonwoven fabric layer Bhaving a lower density than that of the wet type nonwoven fabric layerA.
 2. The multilayer filter medium for use in a filter according toclaim 1, wherein the short-cut nanofibers are ones obtained bydissolving and removing a sea component from a conjugate fibercomprising an island component composed of a fiber-forming thermoplasticpolymer and having an island diameter (D) of 100 to 1,000 nm and a seacomponent composed of a polymer more easily soluble in an alkalineaqueous solution than the fiber-forming thermoplastic polymer.
 3. Themultilayer filter medium for use in a filter according to claim 2,wherein the sea component in the conjugate fiber is polyethyleneterephthalate copolymerized with 6 to 12% by mole of 5-sodium sulfonicacid and 3 to 10% by weight of polyethylene glycol having a molecularweight of 4,000 to 12,000.
 4. The multilayer filter medium for use in afilter according to claim 2, wherein the island component in theconjugate fiber is a polyester.
 5. The multilayer filter medium for usein a filter according to claim 2, wherein the number of islands in theabove-mentioned conjugate fiber is 100 or more.
 6. The multilayer filtermedium for use in a filter according to claim 1, wherein the nonwovenfabric layer B comprises an air-laid nonwoven fabric composed of fibershaving a fiber length of 10 mm or less.
 7. The multilayer filter mediumfor use in a filter according to claim 1, wherein the nonwoven fabriclayer B contains at least 30% by weight or more of binder fibers.
 8. Themultilayer filter medium for use in a filter according to claim 1,wherein the ratio MB/MA of the density (MB) of the nonwoven fabric layerB and the density (MA) of the wet type nonwoven fabric layer A is withinthe range of 0.1 to 0.8.
 9. The multilayer filter medium for use in afilter according to claim 1, wherein the weight ratio of the wet typenonwoven fabric layer A and the nonwoven fabric layer B is within therange of 90/10 to 10/90.
 10. The multilayer filter medium for use in afilter according to claim 1, wherein the nonwoven fabric layer B iscomposed of two or more layers.
 11. A filter using the multilayer filtermedium for use in a filter according to claim 1 and having the nonwovenfabric layer B arranged on a fluid inlet side.
 12. The filter accordingto claim 11, wherein the filter is an air filter for an internalcombustion engine.